This invention relates generally to steelmaking, and particularly carbon steels formed by continuous casting of thin strip.
Thin steel strip may be formed by continuous casting in a twin roll caster. In twin roll casting, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls, which are cooled, so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the rolls to produce a solidified strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the rolls and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting thin strip with a twin roll caster, the molten metal in the casting pool will generally be at a temperature of the order of 1500° C., and usually 1600° C. and above. A high heat flux and extensive nucleation on initial solidification of the metal shells on the casting surfaces is needed to form the steel strip. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial portion of the metal oxides formed are liquid at the initial solidification temperature. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular, International Application AU 99/00641 discloses that a random texture of peaks and troughs in the casting surfaces can enhance initial solidification by providing substantial nucleation sites distributed over the casting surfaces.
Attention has been given in the past to the steel chemistry of the melt, particularly in the ladle metallurgy furnace before the casting of the thin strip. In the past, in U.S. Pat. No. 7,048,033 attention has been given to controlling the oxide inclusions and the oxygen levels in the steel metal and their impact on the quality of the steel strip produced. In U.S. Pat. No. 7,156,151, hydrogen levels and nitrogen levels have been regulated in the molten metal to enhance the casting and quality of the steel strip. In U.S. Pat. No. 6,547,849, a method is disclosed of providing silicon/manganese killed molten steel having a sulfur content of less than 0.02% by weight for casting. Finally, in U.S. patent application Ser. No. 11/622,754, filed Jan. 12, 2007, and published as U.S. 2007/0175608 on Aug. 2, 2007, now abandoned, a thin cast strip with reduced microcracks and method of making the same is disclosed by controlling the sulfur content of the cast strip to between about 0.003% and about 0.008% by weight, along with the carbon content to between about 0.010% and about 0.065% by weight.
In these prior disclosures, the teachings are generally to have low sulfur levels, such as less than 0.025 or 0.02%. See, e.g., International Application AU 99/00641 and U.S. Pat. No. 6,547,849. There is no suggestion of purposely providing very low levels of sulfur to reduce or eliminate microcracking, or for any other purpose, except for U.S. application Ser. No. 11/622,754, filed Jan. 12, 2007, now abandoned. There has been no suggestion to our knowledge of controlling the ratios of manganese/sulfur or manganese/silicon for any reason in the casting of thin strip, or any other steelmaking.
Generally, sulfur has been an undesirable impurity in steelmaking, including in continuous casting of thin strip. Steelmakers generally go to great lengths and expense to minimize sulfur content in making steel. Sulfur is primarily present as sulfide inclusions, such as MnS inclusions. Sulfide inclusions may provide sites for voids and/or surface cracking. Sulfur may also decrease ductility and notch impact toughness of the cast steel, especially in the transverse direction. Further, sulfur creates red shortness, or brittleness in red hot steel. Sulfur also reduces weldability. Sulfur is generally removed from molten steel by a desulphurization process. Steel for continuous casting may be subjected to a deoxidation and then desulphurization in the ladle metallurgy, prior to casting. One such method involves stirring the molten steel by injecting inert gases, such as argon or nitrogen, while the molten metal is in contact with slag having a high calcium content. See U.S. Pat. No. 6,547,849.
On the other hand, thin cast strip formed by twin roll casting has been known to have a tendency to form microcracks in the strip surface. One cause has been the formation of an oxide layer on the surface of the casting rolls that acts as a thermal barrier causing irregular solidification of the cast strip and formation of microcracks in the strip surface.
We have found that microcracking is related to the steel chemistry and certain process parameters. That the “strength” of newly formed shells can be made resistant and reduce the formation of microcracks in the cast strip surface. We have also observed that sulfur is a surface active element in liquid steel. From these observations, we have found that microcracking in the cast strip of low carbon steel can be controlled by regulating the ratio of sulfur to manganese in the molten metal, oxygen and free-oxygen and also to a lesser degree the ratio of manganese to silicon in the molten metal.
The present disclosure describes a thin cast steel strip produced by continuous casting by steps comprising:                a. assembling a pair of internally cooled casting rolls having a nip therebetween and with confining closures adjacent the ends of the nip;        b. introducing molten low carbon steel having a carbon content of between about 0.010% and about 0.065% by weight, less than 5.0% by weight chromium, at least 70 ppm of total oxygen and between 20 and 70 ppm of free oxygen, and an average manganese to sulfur ratio at least 250 between the pair of casting rolls to form a casting pool supported on the casting surfaces of the casting rolls;        c. counter rotating the casting rolls to form solidified metal shells on the casting surfaces of the casting rolls; and        d. forming from said solidified shells thin steel strip downwardly through the nip between the casting rolls.        
The average manganese to silicon ratio in the molten low carbon steel introduced to produce the cast strip may be greater than 3.5.
The thin steel strip produced by continuous casting may have a carbon content between about 0.025% and about 0.065% by weight, or alternatively, a carbon content below about 0.035% by weight.
The thin cast strip may have a chromium content less than 1.5% by weight or less than 0.5% by weight and/or the thin cast strip may have titanium content less than 0.005% by weight.
The thin steel strip may be less than 5 mm in thickness, or less than 2.5 mm in thickness.
The molten metal in the casting pool may have a total oxygen content of at least 100 ppm and a free oxygen content between 30 and 50 ppm. Alternatively or in addition, the thin steel strip produced by continuous casting may be from the molten metal in the casting pool having a nitrogen content less than about 52 ppm. Alternatively or in addition, the sum of the partial pressures of the hydrogen and nitrogen is less than 1.15 atmospheres.
Alternatively, disclosed is a method of casting thin steel strip comprising:                a. assembling a pair of internally cooled casting rolls having a nip therebetween and with confining closures adjacent the ends of the nip;        b. introducing molten carbon steel having a carbon content of between about 0.010% and about 0.065% by weight, less than 5.0% by weight chromium, at least 70 ppm of total oxygen and between 20 and 70 ppm of free oxygen, and an average manganese to sulfur ratio of at least about 250 between the pair of casting rolls to form a casting pool supported on the casting surfaces of the casting rolls;        c. counter rotating the casting rolls to form solidified metal shells on the casting surfaces of the casting rolls; and        d. forming from said solidified shells thin steel strip downwardly through the nip between the casting rolls.        
The average manganese to silicon ratio in the molten low carbon steel introduced in the method to produce cast strip may be greater than 3.5.
A thin steel strip produced by the method of casting steel strip may have a carbon content between about 0.010% and about 0.065% by weight.
The thin cast strip produced by the method may have a chromium content less than 1.5% by weight or less than 0.5% by weight and/or the thin cast strip may have titanium content less than 0.005% by weight.
The thin steel strip may be less than 5 mm in thickness, or less than 2.5 mm in thickness.
We have also found that additional variables that effect solidification and ‘strength’ of the newly formed shells are the temperature of the molten metal in the tundish and casting speed. Reduced temperature of the molten metal in tundish and cast speeds allows time for shell growth to larger thickness and more strength reducing microcracking adjacent to the surface of the cast strip. We have found that the thin steel strip produced by continuous casting may be cast at a tundish temperature for the molten metal below 1612° C. (2933.7° F.) and a casting speed less than 76.88 meters per minute. These additional variables are relevant to both the thin cast strip produced as well as the method by which the thin cast strip is produced.